Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes as precatalysts for direct arylation of azoles with aryl bromides

Article abstract of DOI:10.1016/j.tet.2019.02.031

A series of mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes with general formula [(NHC)Pd(η³-allyl)]₂(μ₂-N-heterocycles)(BF₄)₂ were prepared in one pot based on anion metathesis of (NHC)Pd(η³-allyl)Cl complexes and then ligand replacement with N-heterocycles [N-heterocycles = pyrazine (pyz), 4,4′-bipyridine (bpy) and trans-4,4′-bipyridylethylene (bpe)]. The solid-state structures shown dinuclear structures with two palladium(II) centers holding together by bridged N-heterocycles. Initially investigation of the obtained complexes as precatalysts for direct C[sbnd]H bond arylation of azoles with aryl bromides was carried out.

Full text of DOI:10.1016/j.tet.2019.02.031

Accepted Manuscript
Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes as
precatalysts for direct arylation of azoles with aryl bromides
Jin Yang
PII:
S0040-4020(19)30190-5
https://doi.org/10.1016/j.tet.2019.02.031
TET 30154
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 27 December 2018
Revised Date: 5 February 2019
Accepted Date: 15 February 2019
Please cite this article as: Yang J, Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl
complexes as precatalysts for direct arylation of azoles with aryl bromides, Tetrahedron (2019), doi:
https://doi.org/10.1016/j.tet.2019.02.031.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes as
precatalysts for direct arylation of azoles with aryl bromides
Jin Yang*
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P. R. China;
*Corresponding author: Jin Yang, Tel: (+)86-561-380-2235, Email: yangjinlz@163.com
ABSTRACT
A series of mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl
complexes with general formula [(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycles)(BF4)2 were
prepared in one pot based on anion metathesis of (NHC)Pd(η3-allyl)Cl complexes and
then ligand replacement with N-heterocycles [N-heterocycles = pyrazine (pyz),
4,4`-bipyridine (bpy) and trans-4,4`-bipyridylethylene (bpe)]. The solid-state
structures shown dinuclear structures with two palladium(II) centers holding together
by bridged N-heterocycles. Initially investigation of the obtained complexes as
precatalysts for direct C−H bond arylation of azoles with aryl bromides was carried
out.
Keywords: N-Heterocyclic carbene palladium complexes; N-Heterocycles; Azoles;
C−H bond arylation.
1. Introduction
Since Herrmann demonstrated that NHC‒Pd complexes possess high thermal stability
and are suitable for use in catalysis, considerable effort has been extended to design
and synthesis of various NHC‒Pd complexes.1 Application of the complexes as
catalysts in organic transformations has been well-done, which to further understand
the key structural features that lead to improved catalytic performance.2 However,
previous research has been often conducted within in situ synthesis of NHC‒Pd
catalysts, in which an imdazolium salt is stirred with a base to generate the free
carbene in the presence of a Pd salt.3 The high reactivity of carbenes often leads to
uncertainty about both the rate of catalyst formation and the structure of the catalyst.4
Based on the above reasons, a number of well-defined NHC‒Pd precatalysts with a
1:1 Pd to carbene ratio have been developed and are now commercially available,
such as Ying and Nolan’s NHC-ligated palladacycles,5 Organ’s Pd-PEPPSI-NHC
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precatalysts,6 Nolan’s (NHC)Pd(R-allyl)Cl and (NHC)Pd(acac)Cl systems7. The key
feature of these precatalysts is their effective conversion into catalytically active
NHC−Pd(0) species.8 For example, the good catalytic activities of the
Pd-PEPPSI-NHC complexes are due to the weak coordinating pyridyl ligands can
disassociate from the metal center to form NHC−Pd(0) species and then re-coordinate
to it after the catalytic cycle.9 In order to further explore the catalytic activities of the
NHC–Pd(II) complexes, some heteroatomic compound have been introduced into the
coordination with the palladium centers.10 In contrast, less study has been focused on
the modification of NHC–Pd–allyl complexes with ancillary ligand.11 It is proposed
that the NHC–Pd–allyl complexes in conjunction with an ancillary ligand, which
weakens the Pd–allyl bond energy, can effectively adjust the catalytic activities of the
Pd-precatalyst.
Moreover, organometallic complexes that incorporate more than one metal
center may exhibit unusual catalytic activity due to the cooperative electronic and
steric effects from the ligand and metal centers.12 Because of the electronic and steric
versatility, aromatic N-heterocycles such as pyrazine, 4,4`-bipyridine and
1,2-di(pyridin-4-yl)ethene have been the most widely used bridging ligands to join
two NHC–Pd units to form dinuclear complexes and provide models to test catalytic
activities of the dimetallic catalysts.13 To further perform these studies we revisit
precatalysts of the type (NHC)Pd(η3-allyl)Cl, which were originally reported by
Nolan.7a As an extension of this work, a series of N-heterocycles modified
NHC–Pd–allyl complexes have been synthesized and their application as precatalysts
for arylation of azoles with aryl bromides was initially carried out.
Scheme 1 Synthesis of the dinuclear NHC‒Pd complexes
2. Results and Discussion
2.1 Synthesis of the [(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycle)(BF4)2 complexes
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Following the synthetic route reported by Nolan previously, the (NHC)Pd(allyl)Cl
complexes were easily obtained from reaction of imidazolium salts, [Pd(allyl)Cl]2 and
KOtBu in dry THF [NHC
=
IPr or SIPr; IPr
=
N,N′-bis-
N,N′-bis-
(2,6-di(isopropyl)phenyl)imidazol-2-ylidene;
SIPr
=
(2,6-di(isopropyl)phenyl)imidazolidin-2-ylidene].7a As shown in Table 1, the
[(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycle)(BF4)2 complexes were prepared from
(NHC)Pd(allyl)Cl in one pot based on anion metathesis and then ligand replacement
with N-heterocycles [N-heterocycles = pyrazine (pyz), 4,4`-bipyridine (bpy) and
trans-4,4`-bipyridylethylene (bpe)]. The anion metathesis was carried out using
(NHC)Pd(allyl)Cl (1a and 1b) and AgBF4 as substrates in CH3CN, giving the
corresponding solvated complexes (NHC)Pd(η3-allyl)(CH3CN)BF4, which were
subsequently reacted with N-heterocycles through ligand replacement to afford the
above-mentioned products 2a−2f. Complexes 2a−2f have been fully characterized by
1
1H NMR, 13C NMR, elemental analysis and HR-MS. H NMR analysis of the
complexes is consistent with the twofold symmetry of the complexes, which contains
the stoichiometric proton signal resonances of the NHCs, allyl moieties and
13
N-heterocycles. The C NMR spectra displayed the appearance of carbonic carbons
as diagnostic singlet peaks (180.02‒180.49 ppm for IPr-bearing complexes and
206.36‒207.15 ppm for SIPr-bearing complexes), which were significantly shifted
down-field when compared to the common NHC‒Pd complexes. The results, on the
other hand, indicated that the stronger ϭ-donor ability of the N-heterocycles.
Moreover, crystals of 2a, 2c, 2e and 2f were grown in a mixed solution of
dichloromethane/n-hexane, and their structures were determined using single-crystal
X-ray diffraction.14 As depicted in Figure 1, in all cases the dinuclear structures
consist of bidentate N-heterocycles bridged two palladium centers, which complete
their coordination sphere with NHCs and allyl moieties. The allyl moiety is
η3-coordinated to the palladium with one terminal carbon trans to the carbene and the
second terminal carbon trans to N-heterocycle. The relative disposition of the ligands
about each palladium center is anti, with the two NHCs and two allyl moieties
pointing in opposite directions, respectively. Among the four complexes, the Pd‒CNHC
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bond lengths were in a narrow range of 2.059(4)‒2.079(4) Å and the Pd‒Callyl bond
lengths were in a range of 2.089(8)‒2.203(2) Å, which were slightly longer than that
of the Pd‒CNHC bonds. In addition, the Pd‒N bond lengths, which were in a range of
2.102(6)‒2.155(4) Å, were slightly longer than that of Pd-PEPPSI-NHC complexes.
Figure 1. ORTEP diagrams of 2a, 2c, 2e and 2f with thermal displacement parameter drawn at
30% probability. Hydrogen atoms, anions and solvent molecules have been omitted for clarity.
Symmetry codes: A 1 ‒ x, 1 ‒ y, 2 ‒ z; B 1 ‒ x, 2 ‒ y, 2 ‒ z; C 2 ‒ x, 2 ‒ y, 1 ‒ z; D 2 ‒ x, 1 ‒ y, 2 ‒ z.
2.2 The catalytic activities of the complexes for arylation of azoles with aryl
bromides
With the obtained NHC‒Pd complexes in hand, we then investigated their catalytic
performances in arylation reaction of azoles with aryl bromides, which could be
efficiently catalyzed by some well-defined NHC‒Pd complexes.15 Initially, complex
2a (0.5 mol%) was selected as the model precatalyst, reaction of benzoxazole (0.75
mmol) with 4-bromotoluene (0.5 mmol) as a model reaction to test the reaction
conditions and in particular the solvent and the base. Consider that the ring-opening
reaction of benzoxazole occurred with inorganic base, we increased the ratio of
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benzoxazole to aryl bromide to 1.5:1. As shown in Table 1, the desired product could
be obtained in good yield when the reaction was performed in DMF
(N,N`-dimethylformamide) at 100 °C with LiOtBu as the base. Moreover, moderate
yields were obtained when other inorganic bases, such as NaOtBu, NaOH and KOH
were used. However, KOtBu, K2CO3 and Cs2CO3 are not effective in this catalytic
system. To further optimize the reaction conditions, we examined a series of solvents,
including DMAc (N,N`-dimethylacetamide), DME (dimethoxyethane), dioxane,
tetrahydrofuran (THF) and toluene at 100 °C in the presence of LiOtBu. With regard
to the reaction solvent, DME is found to be the most effective solvent for this process
and that of DMF was comparable as well. The reaction time was also examined and
when reaction time was extended to 24 h, the yield of the product increased slightly.
Furthermore, we also found that the yields of the product were not improved
significantly upon increasing the amount of catalyst loading or the reaction
temperature. On the basis of the screening, DME and LiOtBu were proved to be the
efficient solvent and base, respectively, with 0.5 mol % complex 2a at 100 °C for 12 h,
thus giving the optimal coupling result.
Table 1. Reaction conditions for complex 2a catalysed arylation of benzoxazole with
4-bromotoluenea
Entry NHC‒Pd
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMAc
DME
Base
LiOtBu
NaOtBu
KOtBu
NaOH
KOH
Yieldb (%)
1
2
3
4
5
6
7
8
9
2a (0.5%)
2a (0.5%)
2a (0.5%)
2a (0.5%)
2a (0.5%)
2a (0.5%)
2a (0.5%)
2a (0.5%)
2a (0.5%)
85
80
68
78
75
65
65
82
87
K2CO3
Cs2CO3
LiOtBu
LiOtBu
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10
11
12
2a (0.5%) 1,4-dioxane LiOtBu
60
trace
trace
90
2a (0.5%)
2a (0.5%)
THF
toluene
DME
DME
DME
DME
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
13c 2a (0.5%)
14
15
2a (1.0%)
2a (0.3%)
90
70
16d 2a (0.5%)
89
a
Reaction conditions: 4-bromotoluene (0.50 mmol), benzoxazole (0.75 mmol), complex 2a
b
c
(0.0025 mmol), base (1.0 mmol), solvent (4.0 mL). Isolated yield. The reaction time was
prolonged to 24h. d The reaction temperature was increased to 120 oC.
Next, under the optimal reaction conditions, complexes 2a‒2f catalyzed arylation
of benzoxazoles with a variety of aryl bromides were investigated to compare the
catalytic activities of the complexes. Firstly, benzoxazole was used as one of the
substrates and a variety of aryl bromides were examined for the arylation reaction. As
shown in scheme 2, the position and electronic effect of the substituted groups
influenced the isolated yields of the products to some content. Usually,
electron-withdrawing groups have an effect on increasing the reactivity of substrates.
Aryl bromides with electron-withdrawing electron-donating groups (such as 4-F and
4-Cl) were more reactive than that with an electron-donating substituent in
para-position (4-Me, 4-Et, 4-OMe and 4-OEt), as well as the electron-neutral
bromobenzene. The steric effect of the substituent was also observed. In comparison
with para-substituted bromobenzenes, when using the ortho- and meta-substituted
bromobenzenes, moderate yields of the products were achieved. Subsequently, a
variety of substituted benzoxazoles were surveyed and the results listed in Scheme 2.
5-Me, 5-OMe and 6-Me-substituted benzoxazoles were displayed moderate reactivity
to 4-bromotoluene under the present reaction conditions. On the other hand, the
reaction fails when aryl chloride (such as 4-chlorotoluene) are employed as substrates
(data
not
shown
in
Scheme
2).
Generally,
the
modified
(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycles)](BF4)2 complexes displayed moderate to
6

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good outcomes in arylation of benzoxazoles with aryl bromides, but arylation of aryl
chlorides afforded very low yields.
Scheme 2 Arylation of benzoxazoles with aryl bromides catalysed by 2a‒2f a,b
a Reaction conditions: aryl bromide (0.50 mmol), benzoxazole (0.75 mmol), NHC‒Pd (0.0025
mmol), LiOtBu (1.0 mmol), DME (4.0 mL) at 100 oC for 12 h. b Isolated yield.
To further examine the versatility of this protocol, the arylation of other type azoles
with 4-bromotoluene was carried out under the optimized reaction conditions. It was
found that the arylation reactions with dihydrooxazoles and 1,3,4-oxadiazoles could
afford the corresponding products in good yields in all tested cases (Scheme 3). When
thiazoles were involved in the reaction, the lower yields of the products were obtained.
However, when heteroaromatic aryl bromides such as 2-bromopyridine and
2-bromofuran were used as the substrates, trace amounts of products was observed
(data not shown in Scheme 3). The possible reason was that the substrates bearing
ligating sulfur atoms may decrease the activities of the catalysts and the multidentate
products may coordinate to the NHC−Pd units to form a more stable complexes,
which stunted the transformation of complexes into NHC−Pd(0). Overview, the
results indicated that obtained complexes were still efficient in the catalytic process.
However, this catalytic system has no advantage in comparison with the PEPPSI-type
complex (IPr)PdCl2(1-methylimidazole) reported by Shao, which shown high actives
for (hetero)aryl chlorides.15b With our results in hand, the catalytic cooperativity
between the two metals was not found in our systems due to the long distances
between the two Pd-centers. The catalytic actives of the NHC−Pd complexes depend
7

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on the synergic action of the stability the catalyst, the transformation of complexes
into NHC−Pd(0) and the interaction between the catalyst and the substrates. Moreover,
as can be seen by comparing the catalytic outcomes provided by the different catalysts,
all the synthesized compounds demonstrated similar activities in the arylation
reactions, which indicated the similar ϭ-donor ability of the N-heterocycles. The good
reactivity of the obtained complexes is mainly attributable to the steric and electronic
properties of the NHC moieties. On the other hand, the ancillary coordination of the
N-heterocycles, with a certain degree of tuning the electronic and steric properties of
the metal coordination sphere, affects the conversion of the precatalysts into
catalytically active species in a certain extent.
Scheme 3 Arylation of azoles with 4-bromotoluene catalysed by 2a‒2f a,b
a Reaction conditions: 4-bromotoluene (0.50 mmol), azole (0.75 mmol), NHC‒Pd (0.0025 mmol),
LiOtBu (1.0 mmol), DME (4.0 mL) at 100 oC for 12 h. b Isolated yield.
3. Conclusion
In conclusion, a series of mixed N-heterocycles/N-heterocyclic carbene palladium(II)
allyl complexes with general formula [(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycles)](BF4)2
using pyz, bpy and bpe as ancillary ligands has been developed. Their catalytic
activities were initially tested in arylation of azoles with aryl bromides. Although the
activities of the complexes are lower than some of the well-defined NHC−Pd catalysts
for this reaction, an advantage of the current system is the use of one-pot method to
enter into a range of dinuclear NHC‒Pd complexes. Further modification of the
NHC–Pd complexes and their application are in progress in our laboratory.
4.1 General remarks
The chemicals were purchased from commercial suppliers and were used without
purification prior to use unless otherwise indicated. NMR spectra were recorded at
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400 MHz (for 1H NMR) and 100 MHz (for 13C NMR) on a Bruker Avance 400 NMR
1
spectrometer. All H, and 13C NMR were performed in CDCl3 or CD3CN with
tetramethylsilane (TMS) as an internal standard. The C, H, and N analyses were
performed with a Vario El III elementar. Flash column chromatography was
performed on silica gel (300‒400 mesh).
4.2. Experimental procedure
4.2.1. General procedure for the synthesis of the NHC−Pd complexes
A round-bottom flask equipped with a stir bar was charged with the (NHC)Pd(allyl)Cl
(0.50 mmol, 286 mg), AgBF4 (0.55 mmol, 108 mg) and CH3CN (5.0 mL). The
reaction mixture was stirred at room temperature for 1h and a solution of
N-heterocycles (0.25 mmol) in CH2Cl2 (5.0 mL) was added dropwise with stirring.
The reaction mixture was stirred for a further 4 h. The solvents were removed under
reduced pressure and the residue was purified by flash chromatography on short silica
gel and recrystallized from n-hexane–CH2Cl2 to afford the desired products. Single
crystals of the complexes for X-ray diffraction analysis were obtained from the
evaporation of their n-hexane/CH2Cl2 solutions.
[(IPr)Pd(allyl)]2(ꢀ2-pyz)](BF4)2 (2a)
1
The procedure yielded 300 mg (91%) of the pure 2a as a white powder. H NMR
(CD3CN, 400 MHz): δ (ppm) = 8.51 (s, 4H, CH-pyz), 7.55 (t, J = 8.0 Hz, 4H,
p-CH-aniline), 7.54 (s, 4H, N-CH=CH-N), 7.40 (d, J = 7.6 Hz, 8H, m-CH-aniline),
5.16–5.05 (m, 2H, CH-allyl), 4.24 (d, J = 7.6 Hz, 2H, CH2-allyl), 3.05 (d, J = 13.6 Hz,
4H, CH2-allyl), 2.72 (br, 8H, CH(CH3)2), 1.82 (br, 2H, CH2-allyl), 1.29 (d, J = 6.8 Hz,
13
24H, CH(CH3)2), 1.17 (br, 24H, CH(CH3)2). C NMR (CD3CN, 100 MHz): δ (ppm)
= 180.02 (Ccarbene), 146.27 (Cpyz), 136.31 (Caniline), 131.47 (Caniline), 126.31
(N-CH=CH-N), 125.16 (Caniline), 119.42(Caniline), 77.03 (CH-allyl), 52.68 (CH2-allyl),
29.46 (CH(CH3)2), 23.09 (CH(CH3)2). HR-MS (ESI): calcd for C64H86N6Pd2 [M –
‒ 2+
2BF4 ] 576.2494; found 576.2410. Calcd. for [(IPr)Pd(allyl)]2(ꢀ2-pyz)](BF4)2
(C64H86B2F8N6Pd2): C, 57.98; H, 6.54; N, 6.34%. Found: C, 57.76; H, 6.83; N, 6.61%.
[(SIPr)Pd(allyl)]2(ꢀ2-pyz)](BF4)2 (2b)
1
The procedure yielded 283 mg (85.1%) of the pure 2b as a white powder. H NMR
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(CD3CN, 400 MHz): δ (ppm) = 8.54 (s, 4H, CH-pyz), 7.46 (t, J = 7.6 Hz, 4H,
p-CH-aniline), 7.35 (d, J = 7.6 Hz, 8H, m-CH-aniline), 5.12–5.02 (m, 2H, CH-allyl),
4.21 (d, J = 7.6 Hz, 2H, CH2-allyl), 4.12 (s, 8H, N-CH2-CH2-N), 3.25 (sept, J = 6.8
Hz, 8H, CH(CH3)2), 3.14 (br, 2H, CH2-allyl), 3.00 (d, J = 13.6 Hz, 4H, CH2-allyl),
1.67 (br, 2H, CH2-allyl), 1.37 (d, J = 6.8 Hz, 24H, CH(CH3)2), 1.30 (d, J = 6.4 Hz,
13
24H, CH(CH3)2). C NMR (CD3CN, 100 MHz): δ (ppm) = 206.36 (Ccarbene), 146.26
(Cpyz), 136.60 (Caniline), 130.51 (Caniline), 125.50 (Caniline), 119.97 (Caniline), 77.23
(CH2-allyl), 55.34 (N-CH2-CH2-N), 52.50 (CH2-allyl), 29.29 (CH(CH3)2), 26.37
‒ 2+
(CH(CH3)2), 23.89 (CH(CH3)2). HR-MS (ESI): calcd for C64H90N6Pd2 [M – 2BF4 ]
578.2650; found 578.2658. Calcd. for [(SIPr)Pd(allyl)]2(ꢀ2-pyz)](BF4)2
(C64H90B2F8N6Pd2): C, 57.80; H, 6.82; N, 6.32%. Found: C, 57.58; H, 6.54; N, 6.60%.
[(IPr)Pd(allyl)]2(ꢀ2-bpy)](BF4)2 (2c)
1
The procedure yielded 310 mg (88%) of the pure 2c as a white powder. H NMR
(CDCl3, 400 MHz): δ (ppm) = 7.78 (d, J = 6.4 Hz, 4H, o-CH-bpy), 7.66 (t, J = 8.0 Hz,
4H, p-CH-aniline), 7.52 (d, J = 6.4 Hz, 4H, m-CH-bpy), 7.35 (d, J = 8.0 Hz, 4H,
m-CH-aniline), 7.31 (d, J = 8.0 Hz, 4H, m-CH-aniline), 7.27 (s, 4H, N-CH=CH-N),
5.08 (sept, J = 7.2 Hz, 2H, CH-allyl), 3.83 (d, J = 7.2 Hz, 2H, CH2-allyl), 2.95 (d, J =
13.6 Hz, 2H, CH2-allyl), 2.90 (d, J = 6.8 Hz, 2H, CH2-allyl), 2.82 (sept, J = 6.8 Hz,
4H, CH(CH3)2), 2.68 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 2.71 (d, J = 12.0 Hz, 2H,
CH2-allyl), 1.19 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.14 (d, J = 6.4 Hz, 12H, CH(CH3)2),
1.12 (d, J = 6.4 Hz, 12H, CH(CH3)2), 1.06 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C NMR
(CDCl3, 100 MHz): δ (ppm) = 180.33 (Ccarbene), 152.00 (o-Cbpy), 145.21 (p-Cbpy),
145.16 (Caniline), 135.05(Caniline), 131.25 (Caniline), 124.70 (m-Cbpy), 123.84
(N-CH=CH-N), 118.67 (Caniline), 76.75 (CH-allyl), 50.23 (CH2-allyl), 28.74
(CH(CH3)2), 28.53 (CH(CH3)2), 25.86 (CH(CH3)2), 25.47 (CH(CH3)2), 22.54
‒ 2+
(CH(CH3)2), 22.40 (CH(CH3)2). HR-MS (ESI): calcd for C70H90N6Pd2 [M – 2BF4 ]
614.2650;
found
614.2657.
Calcd.
for
[(IPr)Pd(allyl)]2(ꢀ2-bpy)](BF4)2
(C70H90B2F8N6Pd2): C, 59.97; H, 6.47; N, 5.99%. Found: C, 60.21; H, 6.74; N, 6.18%.
[(SIPr)Pd(allyl)]2(ꢀ2-bpy)](BF4)2 (2d)
1
The procedure yielded 305 mg (87%) of the pure 2d as a white powder. H NMR
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(CDCl3, 400 MHz): δ (ppm) = 7.74 (d, J = 6.4 Hz, 4H, o-CH-bpy), 7.61 (d, J = 6.4 Hz,
4H, m-CH-bpy), 7.51 (t, J = 7.6 Hz, 4H, p-CH-aniline), 7.25 (d, J = 6.8 Hz, 4H,
m-CH-aniline), 7.23 (d, J = 8.0 Hz, 4H, m-CH-aniline), 5.02 (sept, J = 7.2 Hz, 2H,
CH-allyl), 4.11 (s, 8H, N-CH2-CH2-N), 3.78 (d, J = 6.4 Hz, 2H, CH2-allyl), 3.18 (sept,
J = 6.8 Hz, 8H, CH(CH3)2), 2.91 (d, J = 13.6 Hz, 2H, CH2-allyl), 2.83 (d, J = 6.8 Hz,
2H, CH2-allyl), 1.64 (d, J = 12.0 Hz, 2H, CH2-allyl), 1.26 (d, J = 6.0 Hz, 12H,
CH(CH3)2), 1.25 (d, J = 6.4 Hz, 12H, CH(CH3)2), 1.18 (d, J = 6.8 Hz, 12H,
CH(CH3)2), 1.12 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C NMR (CDCl3, 100 MHz): δ
(ppm) = 207.15 (Ccarbene), 152.26 (o-Cbpy), 146.43 (p-Cbpy), 146.36 (p-Cbpy), 145.09
(Caniline), 135.51 (Caniline), 130.19 (Caniline), 124.89 (m-Cbpy), 124.86 (m-Cbpy), 123.75
(Caniline), 119.35(Caniline), 77.17 (CH-allyl), 54.32 (N-CH2-CH2-N), 50.46 (CH2-allyl),
28.67 (CH(CH3)2), 28.49 (CH(CH3)2), 26.41 (CH(CH3)2), 26.19 (CH(CH3)2), 23.31
‒ 2+
(CH(CH3)2), 23.17 (CH(CH3)2). HR-MS (ESI): calcd for C70H94N6Pd2 [M – 2BF4 ]
616.2807; found 616.2817. Calcd. for [(SIPr)Pd(allyl)]2(ꢀ2-bpy)](BF4)2
(C70H94B2F8N6Pd2): C, 59.80; H, 6.74; N, 5.98%. Found: C, 60.23; H, 6.99; N, 6.20%.
[(IPr)Pd(allyl)]2(ꢀ2-bpe](BF4)2 (2e)
1
The procedure yielded 315 mg (88%) of the pure 2e as a white powder. H NMR
(CDCl3, 400 MHz): δ (ppm) = 7.69–7.65 (m, 8H, o-CH-bpe and p-CH-aniline), 7.51
(s, 2H, CH-vinylene), 7.38 (d, J = 9.0 Hz, 4H, m-CH-aniline), 7.35 (d, J = 8.0 Hz, 4H,
m-CH-aniline), 7.27 (br, 8H, m-CH-bpe and N-CH=CH-N), 5.07 (sept, J = 9.0 Hz, 2H,
CH-allyl), 3.74 (d, J = 7.0 Hz, 2H, CH2-allyl), 2.90 (br, 4H, CH2-allyl), 2.84 (sept, J =
6.5 Hz, 4H, CH(CH3)2), 2.68 (sept, J = 6.0 Hz, 4H, CH(CH3)2), 1.72 (d, J = 11.5 Hz,
2H, CH2-allyl), 1.21 (d, J = 6.5 Hz, 12H, CH(CH3)2), 1.16 (d, J = 7.5 Hz, 12H,
CH(CH3)2), 1.14 (d, J = 7.5 Hz, 12H, CH(CH3)2), 1.08 (d, J = 6.0 Hz, 12H,
CH(CH3)2). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 180.49 (Ccarbene), 151.05 (o-Cbpe),
145.18 (p-Cbpe), 135.07 (Caniline), 131.63 (Cvinylene), 131.26 (Caniline), 124.82 (m-Cbpe),
124.72 (N-CH=CH-N), 124.67 (N-CH=CH-N), 124.21 (Caniline), 118.61 (Caniline),
76.26 (CH-allyl), 50.03 (CH2-allyl), 28.79 (CH(CH3)2), 28.60 (CH(CH3)2), 25.90
(CH(CH3)2), 25.50 (CH(CH3)2), 22.65 (CH(CH3)2), 22.50 (CH(CH3)2). HR-MS (ESI):
‒ 2+
calcd for C72H92N6Pd2 [M – 2BF4 ] 627.2729; found 627.2737. Calcd. for
11

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[(IPr)Pd(allyl)]2(ꢀ2-bpe)](BF4)2 (C72H92B2F8N6Pd2): C, 60.56; H, 6.49; N, 5.89%.
Found: C, 60.28; H, 6.75; N, 6.22%.
[(SIPr)Pd(allyl)]2(ꢀ2-bpe](BF4)2 (2f)
The procedure yielded 312 mg (87%) of the pure 2f as a white powder.1H NMR
(CDCl3, 400 MHz): δ (ppm) = 7.54 (d, J = 6.4 Hz, 4H, o-CH-bpe), 7.50 (t, J = 7.6 Hz,
4H, p-CH-aniline), 7.41 (s, 2H, CH-vinylene), 7.30 (d, J = 6.4 Hz, 4H, m-CH-bpe),
7.26 (d, J = 6.4 Hz, 4H, m-CH-aniline), 7.25 (d, J = 6.4 Hz, 4H, m-CH-aniline), 4.99
(sept, J = 6.4 Hz, 2H, CH-allyl), 4.10 (s, 8H, N-CH2-CH2-N), 3.64 (d, J = 6.4 Hz, 2H,
CH2-allyl), 3.16 (sept, J = 6.8 Hz, 8H, CH(CH3)2), 2.84 (d, J = 6.4 Hz, 2H, CH2-allyl),
2.82 (d, J = 6.4 Hz, 2H, CH2-allyl), 1.64 (d, J = 12.0 Hz, 2H, CH2-allyl), 1.26 (d, J =
6.8 Hz, 12H, CH(CH3)2), 1.25 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.18 (d, J = 6.8 Hz,
12H, CH(CH3)2), 1.12 (d, J = 5.8 Hz, 12H, CH(CH3)2). 13C NMR (CDCl3, 100 MHz):
δ (ppm) = 206.92 (Ccarbene), 151.38 (o-Cbpe), 146.44 (p-Cbpe), 145.68 (Caniline), 135.58
(Caniline), 130.18 (Cvinylene), 124.97 (m-Cbpe), 123.92 (Caniline), 119.34 (Caniline), 76.75
(CH-allyl), 54.39 (N-CH2-CH2-N), 50.49 (CH2-allyl), 28.74 (CH(CH3)2), 28.57
(CH(CH3)2), 26.40 (CH(CH3)2), 26.19 (CH(CH3)2), 23.45 (CH(CH3)2), 23.31
‒ 2+
(CH(CH3)2). HR-MS (ESI): calcd for C72H96N6Pd2 [M – 2BF4 ] 629.2885; found
629.2894. Calcd. for [(SIPr)Pd(allyl)]2(ꢀ2-bpe)](BF4)2 (C72H96B2F8N6Pd2): C, 60.39;
H, 6.76; N, 5.87%. Found: C, 60.66; H, 6.48; N, 6.14%.
4.3 General procedure for the [(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycle)(BF4)2
catalyzed arylation of azoles with aryl bromides
A sealable reaction tube was charged with azole (0.75 mmol), aryl bromide (0.50
mmol), [(NHC)Pd(η3-allyl)]2(ꢀ2-N-heterocycle)(BF4)2 (0.0025 mmol), LiOtBu (1.0
o
mmol) and DME (4.0 mL). The mixture was stirred at 100 C for 12 h. After
completion of the reaction, the filtrate was concentrated and the residue was subjected
to purification via column chromatography to give the corresponding product.
2-(p-Tolyl)benzo[d]oxazole: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 8.14 (d, J = 8.0
Hz, 2H), 7.75 (dd, J = 9.2 and 4.0 Hz, 1H), 7.55 (dd, J = 8.8 and 4.0 Hz, 1H),),
7.36–7.30 (m, 4H), 2.42 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 163.26,
150.66, 142.15, 141.98, 129.58, 127.55, 124.80, 124.42, 124.37, 119.79, 110.42,
12

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21.55.
1
2-(4-Isopropylphenyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) =
8.19 (d, J = 8.0 Hz, 2H), 7.77 (dd, J = 9.2 and 4.0 Hz, 1H), 7.57 (dd, J = 8.8 and 3.6
Hz, 1H), 7.38 (d, J = 8.0 Hz, 1H), 7.34 (t, J = 3.6 Hz, 2H), 2.99 (sept, J = 6.8 Hz, 1H),
13
1.30 (d, J = 6.8 Hz, 6H). C NMR (CDCl3, 100 MHz): δ (ppm) = 163.22, 152.78,
150.66, 142.17, 127.69, 126.97, 124.77, 124.69, 124.39, 119.80, 110.42, 34.17, 23.66.
1
2-(4-Fluorophenyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.24
(dd, J = 8.8 and 5.2 Hz, 2H), 7.56 (dd, J = 9.2 and 3.2 Hz, 1H), 7.34 (dd, J = 9.2 and
4.0 Hz, 1H), 7.36‒7.33 (m, 2H), 7.20 (t, J = 8.4 Hz, 2H). 13C NMR (CDCl3, 100
MHz): δ (ppm) = 164.8 (d, J = 251.2 Hz), 162.1, 150.7, 142.1, 129.8 (d, J = 8.8 Hz),
125.1, 124.6, 123.5 (d, J = 3.3 Hz), 120.0., 116.1 (d, J = 22.1 Hz), 110.5.
1
2-(4-Chlorophenyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.19
(d, J = 8.0 Hz, 2H), 7.77 (dd, J = 9.2 and 3.2 Hz, 1H), 7.58 (dd, J = 8.8 and 4.0 Hz,
13
1H),), 7.50 (d, J = 8.4 Hz, 2H), 7.38‒7.34 (m, 2H). C NMR (CDCl3, 100 MHz): δ
(ppm) = 162.06, 150.76, 142.01, 137.76, 129.26, 128.84, 125.67, 125.33, 124.73,
120.09, 110.61.
2-(4-Methoxyphenyl)benzo[d]oxazole: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 8.18
(d, J = 8.8 Hz, 2H), 7.73 (dd, J = 8.4 and 2.0 Hz, 1H), 7.53 (dd, J = 8.4 and 2.0 Hz,
1H), 7.33–7.27 (m, 2H), 7.00 (d, J = 8.8 Hz, 2H), 3.84 (s, 3H). 13C NMR (CDCl3, 100
MHz): δ (ppm) = 163.1, 162.2, 150.6, 142.2, 129.3, 124.5, 124.3, 119.6, 119.5, 114.3,
110.3, 55.3.
1
2-(4-Ethoxyphenyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.17
(d, J = 8.8 Hz, 2H), 7.73 (dd, J = 8.8 and 2.0 Hz, 1H), 7.53 (dd, J = 8.8 and 2.8 Hz,
1H), 7.34–7.28 (m, 2H), 6.99 (d, J = 8.8 Hz, 2H), 4.08 (q, J = 6.8 Hz, 2H), 3.84 (t, J =
6.8 Hz,3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 163.17, 161.69, 150.61, 142.26,
129.31, 124.47, 124.32, 119.53, 119.43, 114.74, 110.29, 63.63, 14.65.
1
2-(m-Tolyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.10 (s, 1H),
8.05 (d, J = 7.6 Hz, 1H), 7.79–7.77 (m, 1H), 7.59–7.56 (m, 1H), 7.41 (t, J = 7.6 Hz,
1H), 7.36–7.34 (m, 3H), 2.45 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 163.2,
13

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150.7, 142.1, 138.7, 132.3, 128.8, 128.2, 127.0, 125.0, 124.7, 124.5, 119.9, 110.5,
21.3.
2-(o-Tolyl)benzo[d]oxazole: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 8.18 (d, J = 7.2
Hz, 1H), 7.83–7.79 (m, 1H), 7.62–7.58 (m, 1H), 7.44–7.39 (m, 1H), 7.38–7.33 (m,
4H), 2.83 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 163.4, 150.3, 142.1, 138.8,
131.8, 130.9, 129.9, 126.3, 126.0, 125.0, 124.3, 120.1, 110.4, 22.1.
1
2-Phenylbenzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.26 (dd, J = 7.6
and 4.0 Hz, 2H), 7.78 (dd, J = 8.8 and 3.2 Hz, 1H), 7.59 (dd, J = 9.2 and 3.2 Hz, 1H
7.53–7.52 (m, 3H), 7.37–7.34 (m, 2H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 163.0,
150.8, 142.1, 131.5, 128.9, 127.6, 127.2, 125.1, 124.5, 120.0, 110.5.
5-Methyl-2-(p-tolyl)benzo[d]oxazole: 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.13 (d,
J = 8.0 Hz, 2H), 7.54 (s, 1H), 7.43 (d, J = 8.0 Hz, 1H), 7.31 (d, J = 8.0 Hz, 2H), 7.13
(d, J = 8.4 Hz, 1H), 2.48 (s, 3H), 2.43 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm)
163.3, 148.9, 142.3, 141.8, 134.2, 129.6, 127.5, 125.9, 124.6, 119.7, 109.8, 21.6, 21.5.
1
5-Methoxyl-2-(p-tolyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) 8.11
(d, J = 8.4 Hz, 2H), 7.43 (d, J = 8.8 Hz, 1H), 7.31 (d, J = 8.0 Hz, 2H), 7.24 (d, J = 2.4
Hz, 1H), 6.92 (dd, J = 8.8 and 2.4 Hz, 1H), 3.86 (s, 3H), 2.43 (s, 3H). 13C NMR
(CDCl3, 100 MHz): δ (ppm) 164.0, 157.3, 145.3, 143.0, 141.9, 129.6, 127.4, 124.5,
113.3, 110.5, 102.8, 55.9, 21.6.
1
6-Methyl-2-(p-tolyl)benzo[d]oxazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.12
(d, J = 8.0 Hz, 2H), 7.62 (d, J = 8.0 Hz, 1H), 7.35 (s, 1H), 7.31 (d, J = 8.0 Hz, 2H),
7.15 (d, J = 8.0 Hz, 1H), 2.49 (s, 3H), 2.42 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ
(ppm) = 162.8, 150.9, 141.7, 140.0, 135.2, 129.5, 127.4, 125,6, 124.6, 119.1, 110.6,
21.7, 21.5.
1
4-Phenyl-2-(p-tolyl)-4,5-dihydrooxazole: H NMR (CDCl3, 400 MHz): δ (ppm) =
7.96 (dd, J = 7.6 Hz, 2H), 7.40‒7.30 (m, 5H), 7.26 (d, J = 8.0 Hz, 2H), 5.39 (t, J = 9.2
13
Hz, 1H), 4.80 (t, J = 9.2 Hz, 1H), 4.28 (t, J = 8.4 Hz, 1H), 2.43 (s, 3H). C NMR
(CDCl3, 100 MHz): δ (ppm) = 164.75, 142.46, 141.88, 129.05, 128.67, 128.37,
127.51, 126.70, 124.69, 74.73, 70.02, 21.53.
1
4,4-Dimethyl-2-(p-tolyl)-4,5-dihydrooxazole: H NMR (CDCl3, 400 MHz): δ (ppm)
14

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= 7.81 (dd, J = 8.0 Hz, 2H), 7.18 (d, J = 8.0 Hz, 2H), 4.07 (s, 2H), 2.36 (s, 3H), 1.36
(s, 6H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 162.10, 141.39, 128.89, 128.09,
125.14, 78.92, 67.35, 28.33, 21.42.
1
2-Phenyl-5-(p-tolyl)-1,3,4-oxadiazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.11
(dd, J = 7.6 and 4.8 Hz, 2H), 8.00 (d, J = 8.4 Hz, 2H), 7.52‒7.50 (m, 3H), 7.31 (d, J =
13
8.0 Hz, 2H), 2.41 (s, 3H). C NMR (CDCl3, 100 MHz): δ (ppm) = 164.60, 164.19,
142.14, 131.48, 129.65, 128.92, 126.75, 123.93, 121.05, 21.53.
1
2,5-Di-(p-tolyl)-1,3,4-oxadiazole: H NMR (CDCl3, 400 MHz): δ (ppm) = 8.01 (d, J
13
= 8.0, 4H), 7.32 (d, J = 8.0 Hz, 4H), 2.43 (s, 6H). C NMR (CDCl3, 100 MHz): δ
(ppm) = 164.44, 142.09, 129.69, 126.79, 121.22, 21.58.
2-(p-Tolyl)benzo[d]thiazole: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 8.07 (d, J = 8.0
Hz, 1H), 7.99 (d, J = 7.6 Hz, 2H), 7.89 (d, J = 8.0 Hz, 1H), 7.48 (t, J = 7.6 Hz, 1H),
7.37 (t, J = 7.6 Hz, 1H), 7.29 (t, J = 7.6 Hz, 2H), 2.43 (s, 3H). 13C NMR (CDCl3, 100
MHz): δ (ppm) = 168.19, 154.19, 141.37, 134.96, 130.99, 129.68, 127.47, 126.19,
124.96, 123.04, 121.52, 21.46.
2-(p-Tolyl)thiazole: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 7.87 (d, J = 8.0 Hz, 2H),
7.85 (d, J = 7.2 Hz, 1H), 7.29 (d, J = 7.2 Hz, 1H), 7.25 (d, J = 8.0 Hz, 2H), 2.40 (s,
3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 168.53, 143.44, 140.13, 130.93, 129.56,
126.43, 118.24, 21.31.
5. Acknowledgments
Financial support from the Scientific Research Project of Anhui Provincial Education
Department (KJ2015TD002) is gratefully acknowledged.
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